pjsua is an open source command line SIP user agent that is used as
the reference implementation for PJSIP and PJMEDIA. It has many
features, such as:

    * Mutiple identities/account registrations
    * Concurrent calls and conference (unlimited number, but only up
      to 254 sources can be mixed to a single destination)
    * Call features: call hold, call transfer (attended or unattended,
      with or without refersub).
    * SIP Presence/SIMPLE with PIDF and XPIDF support. PUBLISH support.
    * Instant messaging and message composing indication
    * DTMF digits transmission/receipt (RFC 2833)
    * WAV file playing, streaming, and recording.
    * Accoustic echo cancellation (AEC).
    * Auto-answer, auto-play file, auto-loop RTP
    * Support SIP UDP, TCP, and TLS transports. Support for DNS SRV
      resolution.
    * NAT traversal with rport and STUN.
    * Tone generation.
    * Codecs: PCMA, PCMU, GSM, Speex (including wideband/16KHz and
      ultra-wideband/32KHz), L16 (8-48KHz, mono or stereo), and iLBC.
    * Adaptive jitter buffer, adaptive silence detection, and packet
      lost concealment audio features.
